Actuators are disclosed which vibrate a thin membrane by an electrostatic force. Sensors are disclosed which detect displacement of a membrane caused by an external force from a change of the electrostatic capacitance. Such a thin membrane is produced by a technique called MEMS (micro electro-mechanical system) employing a semiconductor working technique.
One application of the actuator or the sensor is an ultrasonic transducer, which has both a function of an actuator emitting an ultrasonic wave by vibrating a membrane and a function of a sensor receiving an ultrasonic wave reflected by an object matter by displacement of the membrane in integration. The ultrasonic transducer is constituted of an array of such elements for emitting and receiving signals.
For connecting the electrodes in the arrayed elements with the respective circuits for driving and detection, wiring is formed through the substrate. For example, in one technique, for decreasing the capacitance between the electrode in the elements and the substrate, a reverse bias is applied to a PN junction formed in a lower portion of the electrode (C. H. Cheng, A. S. Ergun, and B. T. Khuri-Yakub, “Electrical through-wafer interconnects with sub-picofarad parasitic capacitance [MEMS packaging]” in Micro-electromechanical Systems Conference, pp. 18-21, 2001). Further, for decreasing the capacitance between the through-wiring and the substrate, a MIS configuration is formed in the wiring portion (see FIGS. 3AA to 3AB of the above-cited reference). Incidentally the capacitance formed between the electrode in the element and the substrate and the capacitance formed between the through-wiring and the substrate are hereinafter referred to as a parasitic capacitance.
In another technique, the parasitic capacitance is decreased by separating spatially the substrate for forming the membrane for the respective elements to utilize the substrate itself as the electrode as well as the through-wiring (X. Zhuang, A. S. Ergun, Y. Huang, I. O. Wygant, O. Oralkan, and B. T. Khuri-Yakub, “Integration of trench-isolated through-wafer interconnects with 2D capacitive micromachined ultrasonic transducer arrays,” Sensors and Actuators A: Physical, vol. 138, pp. 221-229, Jul. 20, 2007, see FIG. 1.)